Surface Functionalization of Graphene with Polymers for ......polymer chains onto the graphene surface. The “grafting from” method relies on the immo‐ bilization of initiators

Chapter 8

Surface Functionalization of Graphene with Polymersfor Enhanced Properties

Wenge Zheng, Bin Shen and Wentao Zhai

Additional information is available at the end of the chapter

http://dx.doi.org/10.5772/50490

1. Introduction

Graphene, a single-atom-thick sheet of hexagonally arrayed sp2 bonded carbon atoms, hasbeen under the spotlight owning to its intriguing and unparalleled physical properties [1].Because of its novel properties, such as exceptional thermal conductivity, [2] high Young’smodulus, [3] and high electrical conductivity,[4] graphene has been highlighted in fabricat‐ing various micro-electrical devices, batteries, supercapacitors, and composites [5, 7]. Espe‐cially, integration of graphene and its derivations into polymer has been highlighted, fromthe point views of both the spectacular improvement in mechanical, electrical properties,and the low cost of graphite [8, 9]. Control of the size, shape and surface chemistry of thereinforcement materials is essential in the development of materials that can be used to pro‐duce devices, sensors and actuators based on the modulation of functional properties. Themaximum improvements in final properties can be achieved when graphene is homogene‐ously dispersed in the matrix and the external load is efficiently transferred through strongfiller/polymer interfacial interactions, extensively reported in the case of other nanofillers.However, the large surface area of graphene and strong van der Waals force among themresult in severe aggregation in the composites matrix. Furthermore, the carbon atoms on thegraphene are chemically stable because of the aromatic nature of the bond. As a result, thereinforcing graphene are inert and can interact with the surrounding matrix mainly throughvan der Waals interactions, unable to provide an efficient load transfer across the graphene/matrix interface. To obtain satisfied performance of the final graphene/polymer composites,the issues of the strong interfacial adhesion between graphene–matrix and well dispersionof graphene should be addressed.

To date, the mixing of graphene and functionalized graphene with polymers covers themost of the published studies, and the direct modification of graphene with polymers is a

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somewhat less explored approach. However, in many cases, to achieve stable dispersions ofgraphene and adequate control of the microstructure of the nanocomposites, non-covalentor covalent functionalization of graphene with polymers may be necessary. The non-cova‐lent functionalization, which relies on the van der Waals force, electrostatic interaction or π-π stacking [10, 12], is easier to carry out without altering the chemical structure of thegraphene sheets, and provides effective means to tailor the electronic/optical property andsolubility of the nanosheets [13]. The covalent functionalization of graphene derivatives ismainly based on the reaction between the functional groups of the molecules and the oxy‐genated groups on graphene oxide (GO) or reduced GO (r-GO) surfaces [14, 15], such as ep‐oxides and hydroxyls on their basal planes and carboxyls on the edges [16]. Compared withnon-covalent functionalization, the covalent functionalization of graphene-based sheetsholds versatile possibility due to the rich surface chemistry of GO/r-GO. However, it shouldbe pointed out that the non-covalent or covalent attachment of graphene to polymer chainscan improve some properties, but may be negative for others, especially those related to themovement of electrons or phonons. Although the functionalization of graphene with poly‐mers is generally attempted with a view to conferring to the polymer new or improvedproperties, the polymer may also prevent the aggregation of the graphene sheets, where thegraphene-polymer size ratio and molecular weight play important roles. For general bibliog‐raphy on typical graphene-based nanocomposites, the reader can consult several mono‐graphs, reviews, and feature articles that summarize the state of the art of the field.

The objective of the present work is to provide a broad overview on the methods developedto non-covalently or covalently bind graphene to polymers. The covalent linking of poly‐meric chains to graphene is at its initial stages and there is significant room for the develop‐ment of new and improved strategies.

2. The precursor of functionalized graphene

As we know, GO is the main precursor for the functionalization of graphene with poly‐mers. It is because that there are multiple oxygen-containing functionalities, such as hydrox‐yl, epoxy and carboxyl groups on GO sheets [16]. GO is usually produced using differentvariations of the Staudenmaier [17] or Hummers [18] method in which graphite is oxi‐dized using strong oxidants such as KMnO4, KClO3, and NaNO2 in the presence of nitricacid or its mixture with sulfuric [19, 20]. For more details about GO, we refer the reader tothe extensive review of GO preparation, structure, and reactivity by Dreyer et al and Zhuet al. [19, 20].

Furthermore, the reduction of GO will remove most, but not all, of the oxygen-containingfunctionalities such as hydroxyl, carboxylic acid and epoxy groups. Therefore, some func‐tionalization reactions are based on the reduced GO. Generally, GO can be exfoliated usinga variety of methods, most commonly by solvent-based exfoliation and reduction in appro‐priate media or thermal exfoliation and reduction [16, 21] In the former route, the hydrophil‐ic nature and increased interlayer spacing of GO facilitates direct exfoliation into solvents

New Progress on Graphene Research208

(water, alcohol, and other protic solvents) assisted by mechanical exfoliation, such as ultra‐sonication and/or stirring, forming colloidal suspensions of “graphene oxide”. The chemical‐ly reduced graphene oxide is produced by chemical reduction of the exfoliated grapheneoxide sheets using hydrazine, [22, 24] dimethylhydrazine, [25] sodium borohydride fol‐lowed by hydrazine, [26] hydroquinone, [ 27] vitamin C, [28] etc. However, the hazardousnature and cost of the chemicals used in reduction may limit its application. The most prom‐ising methods for large scale production of graphene is the thermal exfoliation and reduc‐tion of GO. Thermally reduced graphene oxide can be produced by rapid heating of dry GOunder inert gas and high temperature [29, 31]. Heating GO in an inert environment at1050°C for 30 s leads to reduction and exfoliation of GO, producing low-bulk-density TRGsheets, which are highly wrinkled [32]. In the thermal process, the epoxy and hydroxyl sitesof GO decompose to produce gases like H2O and CO2, yielding pressures that exceed vander Waals forces holding the graphene sheets together, causing the occurrence of exfoliation.

3. Covalent functionalization of graphene with polymers

It is desirable that stronger bonds are usually formed between the graphene and the poly‐mers by covalent functionalization of graphene with polymers. However, it is usually diffi‐cult to realize because ideal graphene lacks functional groups that can be conjugated with.In some cases, when the graphene sheets were exfiolated from GO, incomplete reductionprocess leaves oxygen-containing functionalities that are then available for further function‐alizations. Other covalent functionalization strategies typically involve further disruption ofthe conjugation of the graphene sheets. Although covalent functionalization of graphenewill compromise some of its natural conductivity, this method is still valuable in some caseswhen graphene’s other properties are desirable. More details of graphene functionalizationvia covalent bonds will be discussed below.

3.1. Functionalizations via “grafting from” method

Until now, “grafting to” and “grafting from” techniques have been developed to graft thepolymer chains onto the graphene surface. The “grafting from” method relies on the immo‐bilization of initiators at the surface of graphene, followed by in situ surface-initiated poly‐merization to generate tethered polymer chains. A number of studies of polymer-functionalized graphene by the “grafting from” method have been reported.

3.1.1. Atom transfer radical polymerization (ATRP)

Among the types of “grafting from“ polymerization [33, 46], ATRP is the most widely used,and represents the majority of the studies reported. ATRP is almost certainly chosen becauseit offers the advantages of radical polymerization, that is, a fast initiation process and thedevelopment of a dynamic equilibrium between dormant and growing radicals [47]. In ad‐dition, a wide range of monomers can be polymerized by ATRP with controlled chainlength. Moreover, block copolymers can be prepared by ATRP because of the living radical

Surface Functionalization of Graphene with Polymers for Enhanced Propertieshttp://dx.doi.org/10.5772/50490

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process. Furthermore, ATRP is probably the most practical technique for preparation offunctional polymers because the terminal alkyl halide can be converted to a wide variety offunctionalities by using conventional organic synthetic procedures.

Figure 1. Synthesis of surface-functionalized GO via attachment of an ATRP initiator (a-bromoisobutyryl bromide) fol‐lowed by polymerization of styrene, butyl acrylate, or methyl methacrylate [38].

Lee et al. [38] have reported a new method for attaching polymer brushes to GO sheet usingsurface-initiated ATRP. The hydroxyl groups present on the surface of GO were first func‐tionalized with a wellknown ATRP initiator (a-bromoisobutyryl bromide), and then poly‐mers of styrene, butyl acrylate, or methyl methacrylate were grown directly via a surface-initiated polymerization (SIP) (Figure 1). The authors studied the case of polystyrene (PS)in detail and presented two main conclusions. First, they suggested that the polymer chainlength can be tunable by changing the ratio of monomer and initiator modified GO. Sec‐ond, they reported that the monomer loading can vary the molecular weight of the graft‐ed PS, which was obtained by gel permeation chromatography (GPC) after detaching bysaponification, and the polydispersity was low, which suggested the polymerization pro‐ceeds in a controlled manner. Furthermore, the PS-functionalized GO was shown to sig‐nificantly increase the solubility in N,N-dimethylformamide (DMF), toluene, chloroform,and dichloromethane, improving the processing potential of these materials for applica‐tions in polymer composites.

Fang et al. [43, 44] demonstrated the ability to systematically tune the grafting density andchain length of PS covalently bonded to graphene sheets by combining diazonium addi‐tion and ATRP. After reduction, r-GO was functionalized with 2-(4-aminophenyl) etha‐nol, reacted with 2-bromo-2-methylpropi-onyl bromide (BMPB), and subsequently thepolymerization of styrene was carried out (Figure 2). Their results showed that the poly‐dispersity of the high grafting density sample was more uniform than that of the lowgrafting density, which was attributed to the the degree of functionalization of r-GO sheetswith the initiator because the diazonium coupling to graphene follows an identical radi‐


cal mechanism as that for carbon nanotubes (CNTs). The relaxation of the polymer chainscovalently bonded to the r-GO surface was strongly confined, particularly for segments inclose proximity to the r-GO surface. This confinement effect could enhance the thermalconductivity of the polymer nanocomposites. The significant increases in thermal conduc‐tivity were observed for only 2.0 wt% functionalized r-GO in PS composites. Also, theresulting PS nanocomposites with 0.9 wt% functionalized r-GO revealed around 70% and57% increases in tensile strength and Young’s modulus.

Figure 2. Synthetic routes for achieving controllable functionalization of graphene. 28

Furthermore, Gonçalves et al. [40] developed the use of poly(methyl methacrylate) (PMMA)grafted from carboxylic groups in GO as a reinforcement filler. Here the BMPB initiatorswere immobilized by two esterification reactions: the carboxylic groups of GO were esteri‐fied with ethylene glycol, followed by reacting with BMPB using the same type of reactionas Lee et al. [38]. In this case, the polydispersity of the grafted PMMA, removed from theGO by hydrolysis, was found to be close to unity, once more suggesting a well-controlledprocess irrespective of the under estimated molecular weights. This PMMA-functionalizedGO showed a good solubility in organic solvents such as chloroform and could be used asreinforcement filler in the preparation of PMMA composite films. Due to the strong interfa‐cial interactions between the PMMA-functionalized GO and PMMA matrix caused by thepresence of short PMMA chains covalently bonded to GO, an efficient load transfer from theGO to the matrix was formed, thus improving the mechanical properties of their nanocom‐posites, which were more stable and tougher than pure PMMA and its nanocomposites withunmodified GO (Figure 3). For example, addition of 1 wt% PMMA-functionalized GO clear‐ly led to a significant improvement of the elongation at break, yielding a much more ductile


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and tougher material. In addition, the presence of PMMA-functionalized GO also stabilizedthe nanocomposites increasing the onset of thermal decomposition by around 50 °C.

Figure 3. Load-displacement nanoidentation (left) and stress-strain curves (right) of films of PMMA and its nanocom‐posites with PMMA-functionalized graphene. 40

Yang et al. [42] also took advantage of the carboxylic groups to graft poly-(2-dimethylami‐noethyl methacrylate) (PDMAEMA) onto GO sheets. Here, the BMPB initiators were attach‐ed onto GO sheets by two steps involving amidation reactions. This functionalization of GOwith PDMAEMA not only enhanced the solubility in acidic aqueous solutions (pH = 1), butalso in short chain alcohols. Moreover, this solubility allowed this functionalized-GO to bemixed with spherical particles of poly(ethylene glycol dimethacrylate-co-methacrylic acid)to generate decorated GO sheets.

3.1.2. Other polymerization methods apart from ATRP

Besides the ATRP method, polycondensation, [33] ring opening polymerization [34], rever‐sible addition-fragmentation chain transfer (RAFT) mediated mini-emulsion polymeriza‐tion [35], direct electrophilic substitution [36], and Ziegler–Natta polymerization [37] havealso been used to functionalize the graphene sheets with various polymer chains, whichwill be discussed below.

Wang et al. [33] functionalized GO sheets with polyurethane (PU) by using the polyconden‐sation method. Here GO sheets were reacted with 4,4’-diphenylmethane diisocyanate fol‐lowed by polycondensation of poly(tetramethylene glycol) and ethylene glycol. Thepresence of PU chains linked to GO remarkably improved the dispersion of GO in PU ma‐trix, which was confirmed by the morphological study, and make it compatible with purePU forming strong interfacial interactions that provide an enhanced load transfer betweenthe matrix and the GO sheets thus improving their mechanical properties, as well as theirthermal properties. With the incorporation of 2.0 wt% PU-functionalized GO, the tensilestrength and storage modulus of the PU nanocomposites increased by 239% and 202%, re‐spectively (Figure 4). Furthermore, the nanocomposites displayed high electrical conductivi‐ty, and improved thermal stability of PU was also achieved (Figure 4).


Figure 4. The improved thermal and mechanical propertied of PU/GO nanocomposites [33].

Etmimi et al. [35] investigated the preparation of PS/GO nanocomposites via RAFT mediat‐ed mini-emulsion polymerization. In this process, dodecyl isobutyric acid trithiocarbonate(DIBTC) RAFT agent was attached to the hydroxyl groups of GO through an esterificationreaction (Figure 5). The resultant RAFT-grafted GO was used for the preparation of PS/GOnanocomposites in miniemulsion polymerization. The stable miniemulsions were obtainedby sonicating RAFT-grafted GO in styrene monomer in the presence of a surfactant, fol‐lowed by polymerizing using AIBN as the initiator to yield encapsulated PS-GO nanocom‐posites. The molecule weight and polydispersity of PS in the nanocomposites depended onthe amount of RAFT-grafted GO in the system, in accordance with the features of the RAFTpolymerization method. The thermal stability of the obtained PS/GO nanocomposites wasimproved, which may be attributed to the intercalation of PS into the lamellae of graphite.Furthermore, the increased RAFT-grafted GO significantly resulted in the improved mechan‐ical properties of the nanocomposites. The storage and loss modulus of the nanocompo‐sites were higher than those of the standard PS when the GO loadings reached 3 and 6%,respectively. Oppositely, as RAFT-grafted GO content increased, the Tg values of the sam‐ple decreased. This was attributed to the change in the molecule weight of the PS chains inthe nanocomposites.


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Figure 5. The overall synthesis route for the preparation of RAFT immobilized GO nanosheets [35].

Figure 6. Fabrication of PP/GO nanocomposites by in situ Ziegler-Natta polymerization [37].

Huang et al. [37] reports the first example of preparation of polypropylene/GO (PP/GO)nanocomposites via in situ Ziegler−Natta polymerization. As illustrated in Figure 6, a Mg/Ticatalyst species was immobilized onto GO sheets by reacting with the surface functionalgroups including −OH and −COOH. Subsequent propylene polymerization led to the in situformation of PP matrix, which was accompanied by the nanoscale exfoliation of GO. Inde‐pendent of the opposing nature of the polymer and GO, a good dispersion of GO sheets inPP matrix was observed, which was verified by morphological examination through TEMand SEM observation. Furthermore, high electrical conductivity was discovered with thusprepared PP/GO nanocomposites, this being the only paper reporting conductive materialsprepared by grafting a polymer from graphene sheets. For example, at a GO loading of 4.9


wt %, σc was measured at 0.3 S m−1. We believe that this must originate from a side reactioninvolving the reduction of GO sheets that occurs in one of the synthetic steps.

3.1.3. Irradiation-induced polymerization

Ultrasound has found important applications in a diverse range of materials and chemi‐cal syntheses [48, 51]. Both the physical and chemical effects of ultrasound arise from acous‐tic cavitation: the formation, growth, and collapse of bubbles in liquids irradiated with highintensity ultrasound [48 - 50, 52]. Localized hot spots with temperature of ~5000K andpressures of hundreds of bars are generated during the bubble collapse within liquid, whichcan induce some chemical reactions that can’t take place under normal conditions. Xu et al.[53] reported a convenient single-step sonication-induced approach for the preparation ofpolymer functionalized graphenes from graphite flakes and a reactive monomer, styrene.In this work, they showed that by choosing a reactive medium as the solvent, the com‐bined mechanochemical effects of high intensity ultrasound can, in a single step, readilyinduce exfoliation of graphite to produce functionalized graphenes. Ultrasonic irradiationof graphite in styrene results in the mechanochemical exfoliation of graphite flakes to single-layer and few-layer graphene sheets combined with functionalization of the graphene withPS chains (Figure 7). The PS chains are formed from sonochemically initiated radical poly‐merization of styrene. They also tested a variety of other solvents, including toluene, ethyl‐benzene, 1-dodecene, and 4-vinylpyridine to prepare functionalized graphenes. Only theeasily polymerizable reactants containing vinyl groups, styrene and 4-vinylpyridine, leadto stable functionalized graphene. Such functionalized graphene have good stability andsolubility in common organic solvents and have great potential for graphene-based compo‐site materials.

Moreover, direct photografting reactions of vinyl monomers came into focus for the prepa‐ration of stable polymer brushes. Two approaches in particular, the sequential “living” pho‐topolymerization and the self-initiated photografting and photopolymerization (SIPGP),attracted the attention of numerous research groups because of the facile preparation andbroad applicability. Steenackers et al. [54] showed that PS chains could covalently bound tographene by the UV-induced polymerization of styrene (Figure 8). Photopolymerization oc‐curs at existing defect sites and that there is no detectable disruption of the basal plane con‐jugation of graphene. This method thus offers a route to define graphene functionalitywithout degrading its electronic properties. Furthermore, photopolymerization with styreneresults in self-organized intercalative growth and exfoliation of few layer graphene sheets.Under these reaction conditions, a range of other vinyl monomers exhibits no reactivity withgraphene. However, the authors demonstrate an alternative route by which the surface reac‐tivity can be precisely tuned, and these monomers can be locally grafted via electron-beam-induced carbon deposition on the graphene surface.


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Figure 7. Experimental setup of the one-step mechanochemical process for exfoliation of graphite and sonochemicalfunctionalization of graphene [53].

Figure 8. Patterned polymer brush layers on CVD-grown single layer graphene are prepared by UV illumination through

a mask in bulk styrene. Surface photopolymerization occurs selectively in illuminated regions of the material [54].

Figure 9. The preparation of PVAc grafted GO by γ-ray irradiation-induced graft polymerization [55].

γ-ray radiation-induced graft polymerization has many advantages, including being a single-step chemical reaction, needing no additives or catalysts, being conducted at room temper‐ature, cost-effective, and so on. Above all, it is versatile for vinyl monomers that undergofree radiation polymerization, and production can be easily scaled-up. Zhang et al. [55]reported a facile approach to functionalize GO sheets with poly(vinyl acetate) (PVAc) by


γ-ray irradiation-induced graft polymerization (Figure 9). Due to the full coverage of PVAcchains and solvated layer formation on GO sheets surface, which weakens the interlami‐nar attraction of GO sheets, PVAc-functionalized GO was well dispersed in common organ‐ic solvents, and the dispersions obtained were extremely stable at room temperature withoutany aggregation.

Furthermore, Lee at al. [56] have developed a method to selectively fluorinate graphene byirradiating fluoropolymer-covered graphene with a laser (Figure 10). Here the sp2-hy‐brized graphene would react with the active fluorine radicals, which was produced byphoton-induced decomposition of the fluoropolymer under laser-irradiation, and form C-F bonds. However, this reaction only occurred in the laser-irradiated region. The kinetics ofC–F bond formation is dependent on both the laser power and fluoropolymer thickness.Furthermore, the resistance of the graphene dramatically increased due to the fluorina‐tion, while the basic skeletal structure of the carbon bonding network is maintained. Thisis an efficient method for isolating graphene devices because the laser irradiation on fluoro‐polymer-covered graphene process produces fluorinated graphene with highly insulatingproperties in a single step.

Figure 10. The scheme showing a mechanism for fluorination under laser-irradiation.56

3.2. Functionalizations via “grafting to” method

As commented previously, the “grafting from” method relies on the immobilization of ini‐tiators at the surface of graphene, followed by in situ surface-initiated polymerization togenerate tethered polymer chains. However, this may not be possible in certain cases, wherethe covalent linkage between the presynthesized polymer and graphene emerges as the onlyalternative. In order to expand the type of polymers that can be bound to graphene, the cate‐gory of “grafting-to” method can be employed to achieve this purpose. The “grafting to”technique involves the bonding of preformed end functionalized polymer chains to the sur‐face of graphene. Therefore, the prepared graphene require adequate functional groups,which could react with specific polymers. Or the polymer has functionalities capable of re‐acting with either graphene or its chemically broader cousin, GO. In the following part, wewill summarize the type of reactions and the families of polymers that have been grafted tothe graphene.


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3.2.1. Esterification/amidation reactions

Esterification/amidation reactions between carboxylic groups in GO and hydroxyl or aminegroups in the polymer have been widely investigated [14, 15, 44, 57, 60]. In this respect,poly(vinyl alcohol) (PVA) was covalently bonded to GO [14, 57] and r-GO [14] by using atypical catalytic system for esterification (Figure 11). After functionalization with PVAchains, the solution processability of graphene was significantly improved. And the degreeof functionalization was shown to be low, probably due to steric hindrance caused by thehuge volume of GO. However, due to the presence of the huge graphene sheets, significantchanges in the crystalline properties as well as in the tacticity of the polymer were observed.The originally semicrystalline PVA became completely amorphous, and the Tg increased by35 °C after bonded to GO sheets. The decrease in crystallinity was attributed to the intercala‐tion of PVA chains between the graphene sheets as well as the formation of “secondary”bonds, for example, hydrogen bonding that breaks intra- and interchain bonds. Finally, ithas been demonstrated that the reaction is favoured at specific conformations at the isotacticsequences where the hydroxyl groups are more exposed (lower internal steric hindrance)than in the syndiotactic counterpart.

Figure 11. Schematic Illustration of the Esterification of GO with PVA [14].

A similar strategy has been approached to functionalize r-GO with poly(vinyl chloride)(PVC) [59]. In this step, the susceptible groups in PVC chains could react with the functionalgroups on r-GO sheets by esterification, which was provided by a nucleophilic substitutionreaction [61, 62]. Furthermore, several methodologies to prepare r-GO/PVC nanocompositesand the optimum conditions have been established. The covalent attachment of r-GO to ap‐propriately functionalized PVC is the only effective method to produce nanocompositeswith improved thermal and mechanical properties (Figure 12). The absolute values of themechanical and thermal properties of PVC-functionalized GO nanocomposites are higherthan those for a similar system using MWNTs as reinforcement because of the higher aspectratio of the r-GO sheets with respect to the MWNTs. The introduction of r-GO also increasesthe Tg of the composites, reflecting the changes in the mobility of the PVC chains. However,due to the lower strength of the “secondary bonds”, such as halogen bonding and hydrogenbonding for PVC and PVA respectively, the changes in Tg for PVC were much lower thanthose reported for PVA, which had a similar degree of functionalization with PVC-function‐


alized GO. The existence of these secondary bonds can lead to some additional ordering thatalters the segmental mobility and consequently the final properties.

Figure 12. Comparison of (a) storage modulus and (b) tan δ curves for neat PVC (square), PVC functionalized CNTs(triangle), PVC functionalized GO (solid circle), and PVC functionalized r-GO (open circle) [59].

Furthermore, conjugated polymer-functionalized graphene materials have also been pre‐pared by esterification/amidation reactions [60, 63]. In these cases, the ends of the conjugat‐ed polymers were bonded to the functional groups on the graphene sheets. As a result, thesolubility of the obtained functionalized graphene was significantly improved in commonsolvents, enabling device preparation by solution processing. Thus, GO functionalized withboth triphenylamine-based polyazomethine-modified GO (TPAPAM-GO) and poly(3-hex‐ylthiophene) modified GO (P3HT-GO) (Figure 13) can be incorporated into specific devicesby simple spin coating to obtain composites that exhibit non-volatile memory effect as wellas higher power conversion efficiency for solar cells, demonstrated in the cases of TPAPAM-GO and P3HT-GO, respectively.

Figure 13. Synthesis procedure for chemical grafting of CH2OH-terminated P3HT chains onto graphene, which in‐volves the SOCl2 treatment of GO (step 1) and the esterification reaction between acyl-chloride functionalized GO andMeOH-terminated P3HT (step 2) [47].


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3.2.2. Nitrene cycloaddition

Nitrene chemistry, an approach used to functionalize graphene with single molecules [64,65], has also been extended to polymers [66, 67].Nitrene chemistry is a versatile tool that al‐lows the functionalization of graphene with a pool of functionalities, potentiating graphenesolubility, and dispersion in a wide variety polymeric matrices. By using of this technology,He and Gao [67] have reported a general and versatile approach to graft of polymers ontographene sheets. In their experiment, a wide range of immobilized functional groups wereused to graft specific polymers from it (Figure 14). Though the cycloadditions of nitrene rad‐icals and thermal reduction of GO occurs simultaneously, the conductivity of graphene di‐minished after functionalization, values of around 300-700 S/m for PS and poly(ethyleneglycol) (PEG)-functioanlzied graphene were obtained. This was mainly due to the highamounts of graphene in the final product, which is reasonable because although the poly‐mer is linked to graphene by the ends, the molecular weight of the polymers is low, makingthe mass percentage of graphene high.

Figure 14. General strategy for the preparation of functionalized graphene sheets by nitrene chemistry and the fur‐ther chemical modifications [67].

Besides the methods mentioned above, other “grafting to” approaches have also been inves‐tigated, such as the opening of maleic rings in maleic acid (MA) grafted polyethylene byamine functionalized graphene [68], nucleophilic epoxy-ring opening in GO by aminegroups in biocompatible poly-l-lysine [69], atom transfer nitroxide radical coupling(ATNRP) of PNIPAM and 2,2,6,6-tetramethylpiperidine-1-oxyl-modified graphene [70], andsimultaneous reduction of GO and radical grafting of PMMA by phase transfer [71].

3.2.3. Irradiation-induced radical grafting

Shen et al. [72] have reported PVA-functionalized graphene (f-G) could be prepared by ul‐trasonication of pristine graphene (p-G) in a PVA aqueous solution (Figure 15). Ultrasonic


irradiation of graphene can cause a considerable amount of defects on the graphene surface,which might produce reactive sites in situ as a result of the high temperature and pressureduring bubble collapse. Moreover, the original defects on pristine graphene can also be easi‐ly destroyed and produce reactive sites during ultrasonic irradiation. The PVA chain radi‐cals produced by sonochemical degradation of the PVA solution can react easily withgraphene, because of the reactive sites formed on the graphene surface, and readily func‐tionalized them via “grafting to” method. The content of PVA on graphene was estimated tobe ~35%. The f-G could be well dispersed in the PVA matrix by a simple solution mixingand casting procedure. Due to the effective load transfer between f-G and PVA matrix, themechanical properties of the f-G/PVA films were significantly improved. Compared withthe p-G/PVA films, a 12.6% increase in tensile strength and a 15.6% improvement of Young’smodulus were achieved by addition of only 0.3 wt% f-G. Moreover, this simple ultrasonica‐tion technique could enable us to functionalize graphene with other polymers.

Figure 15. Schematic illustration of the sonochemical preparation process of PVA-functionalized graphene [72].

As a summary, “grafting-to” methods are highly versatile since they take advantage of thechemistry of GO that can be appropriately modified with a wide variety of functionalgroups providing a capacity for reaction with almost any type of polymers. In addition,“grafting-to” method allows the selection of the location of graphene, that is, at the end or aspart of the main chain that can be directly related to changes in the final properties.

4. Non-covalent functionalization of graphene with polymers

As shown in the aforementioned examples, the covalent functionalization of polymers ongraphene-based sheets holds versatile possibility due to the rich surface chemistry of GO/r-GO. Nevertheless, the non-covalent functionalization, which almost relies on hydrogenbonding or π–π stacking, is easier to carry out without altering the chemical structure of thecapped r-GO sheets, and provides effective means to tailor the electronic/optical propertyand solubility of the nanosheets. The first example of non-covalent functionalization of r-GOsheets was demonstrated by the in situ reduction of GO with hydrazine in the presence of


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poly(sodium 4-styrenesulfonate) (PSS) [12], in which the hydrophobic backbone of PSS sta‐bilizes the r-GO, and the hydrophilic sulfonate side groups maintains a good dispersion ofthe hybrid nanosheets in water.

4.1. Functionalizations via π–π stacking interactions

π–π stacking interactions usually occur between two relatively large non-polar aromaticrings having overlapping π orbitals. They can be comparable to covalent attachment instrength and hence provide more stable alternatives to the weaker hydrogen bonding, elec‐trostatic bonding and coordination bonding strategies. Furthermore, π–π stacking function‐alization does not disrupt the conjugation of the graphene sheets, and hence preserves theelectronic properties of graphene.

4.1.1. Polymers with pyrene end-groups

In order to functionalize graphene with polymers via π–π stacking, one strategy is for thepolymer chains to be synthesized with pyrene moieties as the termini of the polymer chains.RAFT polymerization can be a useful tool to achieve this aim. Polymers with pyrene end-groups have been made using RAFT mechanism in several recent papers [73, 80].

Figure 16. A schematic depicting the synthesis of pyrene-terminated PNIPAAm using a pyrene-functional RAFT agentand the subsequent attachment of the polymer to graphene [ 81].


Liu et al. synthesized thermoresponsive graphene-polymer nanocomposites. They first tookadvantage of RAFT polymerization to synthesize a well-defined thermoresponsive pyreneterminated poly(N-isopropylacrylamide) (PNIPAAm), followed by attachment onto the bas‐al plane of graphene sheets via π-π stacking interactions (Figure 16) [81]. The lower criticalsolution temperature (LCST) of pyrene-terminated PNIPAAm was measured to be 33 ° C.However, after the pyrene-functional polymer functionalized with graphene sheets, the re‐sultant graphene composites were also thermoresponsive in aqueous solutions, but with alower LCST of 24 °C.

Similarly, Liu et al. also prepared pH sensitive graphene-polymer composites by functionali‐zation of graphene with a pyrene-terminated positive charged polymer, poly(2-N,N‘-(di‐methyl amino ethyl acrylate)) (PDMAEA), and a negatively charged polymer, polyacrylicacid (PAA) [80]. During the process, a pyrene-terminated RAFT agent was used to preparethe pyrene-terminated PDMAEA and PAA. When manipulating the pH of the graphene−composite suspensions, phase transfer between the aqueous and organic phases was ob‐served. Self-assembly of the two oppositely charged graphene-polymer composites affordedlayer-by-layer (LbL) structures as evidenced by high-resolution scanning electron microsco‐py (SEM) and quartz crystal microbalance (QCM) measurements (Figure 17). In addition toRAFT mechanism, π-orbital rich polymers have also been synthesized by using of ATRPmethod for functionalization of r-GO to afford the fluorescent and water-soluble graphenecomposites via π-π stacking interactions [82].

Figure 17. Synthesis of pH sensitive pyrene-polymer composites via π-π stacking interactions for the self-assembly offunctionalized graphene into layered structures [80].

4.1.2. Conjugated polyelectrolytes

Moreover, conjugated polyelectrolytes with various functionalities have been used to modi‐fy r-GO nanosheets [83 - 85], in the hope to achieve good solubility in different kinds of sol‐


223

vents, and at the same time acquire added optoelectronic properties. Qi et al. has speciallydesigned an amphiphilic coil–rod–coil conjugated triblock copolymer (PEG-OPE, chemicalstructure shown in Figure 18A) to improve the solubility of graphene-polymer nanocompo‐sites in both high and low polar solvents [33, 83]. In the proposed configuration, the conju‐gated rigid-rod backbone of PEG-OPE can bind to the basal plane of the r-GO via the π-πstacking interaction (Figure 18B), whereas the lipophilic side chains and two hydrophiliccoils of the backbone form an amphiphilic outer-layer surrounding the r-GO sheet. As a re‐sult, the obtained r-GO sheets with a uniformly coated polymer layer (Figure 18C) are solu‐ble in both organic low polar (such as toluene and chloroform) and water-miscible highpolar solvents (such as water and ethanol).

Figure 18. A) Chemical structure of PEG-OPE. (B) Schematic illustration of fabrication of PEG-OPE stabilized r-GOsheets. (C) Tapping-mode AFM image and cross-sectional analysis of PEG-OPE-r-GO on mica [67].

Figure 19. A) Chemical structure of the newly designed PFVSO3. B) Schematic illustration of the synthesis of PFVSO3-stabilized r-GO in H2O: step 1, oxidative treatment of graphite (gray-black) yields single-layer GO sheets (brown); step2, chemical reduction of GO with hydrazine in the presence of PFVSO3 produces a stable aqueous suspension ofPFVSO3-functionalized r-GO sheets (PFVSO3-r-GO). C) Photograph of aqueous dispersions of GO (i), r-GO (ii), PFVSO3-r-GO (iii), and PFVSO3 (iv) [84].


In another study, Qi et al. demonstrated the preparation of highly soluble r-GO hybrid ma‐terial (PFVSO3-r-GO) by taking advantage of strong π–π interactions between the anionicCPE and r-GO (Figure 19) [84]. The resulting CPE-functionalized r-GO (PFVSO3-r-GO)shows excellent solubility and stability in a variety of polar solvents, including water, etha‐nol, methanol, dimethyl sulfoxide, and dimethyl formamide. The morphology of PFVSO3-r-GO is studied, which reveal a sandwich-like nanostructure. Within this nanostructure, thebackbones of PFVSO3 stack onto the basal plane of r-GO sheets via strong π–π interactions,while the charged hydrophilic side chains of PFVSO3 prevent the rGO sheets from aggregat‐ing via electrostatic and steric repulsions, thus leading to the solubility and stability ofPFVSO3-rGO in polar solvents. Furthermore, the presence of PFVSO3 within r-GO inducesphotoinduced charge transfer and p-doping of r-GO. As a result, the electrical conductivityof PFVSO3-r-GO is not only much better than that of GO, but also than that of the unfunc‐tionalized r-GO.

4.1.3. π–π stacking induced by melt blending

High temperature and strong shear forces are usually involved during the melt blendingprocess, which tends to fracture the nanoparticle aggregates, and endow polymer chainswith the ability to diffuse into the gaps of the nanoparticle interlayer. Furthermore, assuggested by theoretical and experimental studies [86, 87] chemical or physical interac‐tions can be formed between the fillers and the polymer components. Zhang et al [88] foundthe melt blending led to enhanced interactions between PS and CNTs, which was indicat‐ed by increased amount of PS linked to CNTs and therefore dramatically increased solubil‐ity of CNTs in some solvents. Taking advantage of this method, Zhou et al [89] obtainedPS-coated CNTs through simple melt mixing of PS with CNTs. Furthermore, Lu et al [90]studied the styrene-butadiene-styrene tri-block copolymer (SBS)/CNTs composite, and theirresults showed that there were interactions between CNTs and SBS occurred during meltmixing, leading to an improvement of the mechanical properties of SBS/CNTs composites,as well as the homogeneous dispersion of CNTs in SBS. The mechanism of melt blendingon these enhanced interactions was mainly attributed to the formation of π-π stackingbetween the aromatic system of π-electrons of PS and the π-electrons system of CNTs duringmelt blending [10, 88].

Melt Blending can also graft PS chains onto the surface of graphene sheets via π-π interac‐tions. The interaction between graphene and PS was significantly enhanced by melt blend‐ing, which led to an increased amount of PS-functional graphene (PSFG) exhibiting goodsolubility in some solvents [10]. The mechanism for the in-situ formation of π-π stackingwas addressed, as illustrated in Figure 20. It was proposed that the strong shear action ap‐plied by extruder could stretch the PS chains and endow the polymer chains with possibilityto diffuse into the interlayer gap of graphene sheets. Moreover, the PS chains could be push‐ed towards to graphene sheets to form the π-π stacking under high shear forces. The UV-visabsorption spectroscopy of PSFG presented an obvious red shift, suggested the presence ofπ-π stacking between PS and graphene.


225

Figure 20. Schematic for the forming of π-π stacking in the process of melt blending [10].

4.2. Functionalizations via hydrogen bonding

Hydrogen bonding is very common in the biological world. An individual hydrogen bond isnot very strong (2–8 kcal/mol); however, multiple hydrogen bonds will afford strong inter‐actions as seen in DNA hybridization. Liang et al. prepared PVA nanocomposites with gra‐phene by dispersing GO sheets into PVA matrix at molecular level [91]. The authorsconsidered that the increased tensile strength and Young's modulus of the PVA/GO compo‐site films were caused by the strong hydrogen bonding interactions between the residualoxygen-containing groups of GO sheets, such as epoxides and hydroxyls on their basalplanes and carboxyls on the edges, and hydroxyl groups of the PVA chains. Polymer/graphene nanocomposites with other hydrophilic polymers, epoxy, poly(acrylonitrile) andpolyaniline exhibited extraordinary high increase in modulus or glass transition tempera‐ture, attributed to hydrogen bonding interactions [92 - 94].

5. Applications of functionalized graphene

The use of graphene or functionalized graphene materials usually exploits properties suchas the large surface area or high electrical conductivity. Currently, the applications of func‐tionalized graphene are focused on clean energy devices and electronic devices, as well ason sensors, medical devices and catalysis. Two examples will be discussed below.

Non-covalent chemical modification between functionalized pyrene and graphene wasadopted to achieve patterned arrays of glucose oxidase (GOD) for potential applications inglucose sensors, cell sensors and tissue engineering [96]. In research by Zeng et al. as illus‐trated in Figure 21, [95] chemically reduced GO was functionalized by pyrene-graftedpoly(acrylic acid) (PAA) in aqueous solution on the basis of π-π stacking as well as van derWaals interactions. Then PAA-functionalized graphene (PAA-graphene) was LbL assem‐bled with poly(ethyleneimine) (PEI). Graphene multilayer films facilitated the electrontransfer, enhancing the electrochemical reactivity of H2O2. On the basis of this property, they


fabricated a bienzyme biosensing system for the detection of maltose by successive LbL as‐sembly of functionalized graphene, GOD, and glucoamylase (GA), which showed greatpromise in highly efficient sensors and advanced biosensing systems.

Figure 21. Schematic Illustration of the Strategies for layer-by-layer assembly of graphene multilayer films for enzyme-based glucose and maltose biosensing [95].

Figure 22. Schematic illustration of the electron-withdrawing from graphene by PDDA to facilitate the ORR process[97].


227

The planar structure and superb conductivity of graphene also provide an appropriate plat‐form for novel electrochemical sensors. A metal-free electrocatalyst for the oxygen reductionreaction (ORR) was achieved with graphene sheets functionalized with an electron acceptor,poly(diallyldimethylammonium chloride) (PDDA) (Figure 22). The resultant positivelycharged graphene composite was demonstrated to show remarkable electrocatalytic activitytoward ORR with better fuel selectivity, tolerance to CO poisoning, and long-term stabilitythan that of the commercially available Pt/C electrode [97]. The observed ORR electrocata‐lytic activity induced by the intermolecular charge-transfer provides a general approach tovarious carbon-based metal-free ORR catalysts for oxygen reduction.

6. Conclusion

Generally speaking, functionalization of graphene with polymers can be achieved via ei‐ther covalent or non-covalent interactions. Furthermore, the functionalization of graphene isalways based on the graphene from previously prepared GO, which has multiple oxygen-containing functionalities, such as hydroxyl and carboxyl groups. In the covalent functional‐izations, “grafting to” and “grafting from” techniques have been developed to graft thepolymer chains onto the graphene surface. The “grafting from” method relies on the immo‐bilization of initiators at the surface of graphene, followed by in situ surface-initiated poly‐merization to generate tethered polymer chains. The “grafting to” technique involves thebonding of preformed end functionalized polymer chains to the surface of graphene. Com‐paring both methods through the examples described above, it seems that “grafting-to“method allows the covalent bonding of a wider variety of polymers to graphene. The poly‐mers “grafted from“ graphene are those produced principally by some type of radical poly‐merization, such as ATRP and RAFT. However, the most relevant polymers grafted fromgraphene, such as PS and PMMA have also been attached to graphene by the “grafting-to“method. In principle, for polymers obtained by ATRP the “grafting-from“ method might bethe most appropriate, but this depends on the features desired in the final composite, be‐cause through “grafting to“ method graphene forms part of the bulk polymer whereas in the“grafting from“ it is limited to a terminal group. Moreover, except for a few exceptions,functional polymers and polymers synthesized by condensation reactions are principallybound to graphene by the grafting-to approach. Regarding to use PVA and PVC to function‐alize graphene, we select the “grafting to“ method due to the experimental procedures em‐ployed to obtain these polymers. In fact, it is well known that PVA is not prepared bypolymerization of the corresponding monomer, and the majority of PVC is synthesized byfree-radical polymerization through suspension or bulk processes. Finally, radiation-in‐duced graft polymerization has been utilized in the “grafting to” and “grafting from” tech‐niques, due to its advantages, including being a single-step chemical reaction, needing noadditives or catalysts, being conducted at room temperature, cost-effective, and so on.

The covalent functionalization of polymers on graphene-based sheets holds versatile possi‐bility due to the rich surface chemistry of GO/r-GO. Nevertheless, the non-covalent func‐tionalization, which almost relies on hydrogen bonding or π-π stacking, is easier to carry


out without altering the chemical structure of the capped rGO sheets, and provides effectivemeans to tailor the electronic/optical property and solubility of the nanosheets. π-π stackinginteractions usually occur between two relatively large non-polar aromatic rings havingoverlapping π orbitals. In order to functionalize graphene with polymers via π-π stacking,one strategy is for the polymer chains to be synthesized with pyrene moieties as the terminiof the polymer chains. Moreover, conjugated polyelectrolytes with various functionalitieshave been used to functionalize graphene via π-π stacking. The hydrogen bonding interac‐tions between the residual oxygen-containing groups of graphene sheets and hydroxylgroups of the hydrophilic polymer chains, such as PVA chains, have also been used to func‐tionalize the graphene in order to obtain increased tensile strength and Young's modulus.

Author details

Wenge Zheng*, Bin Shen and Wentao Zhai

*Address all correspondence to: [email protected]

Ningbo Key Lab of Polymer Materials, Ningbo Institute of Material Technology and Engi‐neering, Chinese Academy of Sciences, China

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Chapter 8Surface Functionalization of Graphene with Polymers for Enhanced Properties1. Introduction2. The precursor of functionalized graphene3. Covalent functionalization of graphene with polymers3.1. Functionalizations via “grafting from” method3.1.1. Atom transfer radical polymerization (ATRP)3.1.2. Other polymerization methods apart from ATRP3.1.3. Irradiation-induced polymerization

3.2. Functionalizations via “grafting to” method3.2.1. Esterification/amidation reactions3.2.2. Nitrene cycloaddition3.2.3. Irradiation-induced radical grafting

4. Non-covalent functionalization of graphene with polymers4.1. Functionalizations via π–π stacking interactions4.1.1. Polymers with pyrene end-groups4.1.2. Conjugated polyelectrolytes4.1.3. π–π stacking induced by melt blending

4.2. Functionalizations via hydrogen bonding

5. Applications of functionalized graphene6. ConclusionAuthor detailsReferences

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Surface Functionalization of Graphene with Polymers for ......polymer chains onto the graphene surface. The “grafting from” method relies on the immo‐ bilization of initiators